The advent of digital recording techniques has enabled digital copies of digital content carried by recording media such as optical discs (for example DVDs) to be made with little or no loss of copy quality. This makes it easy for an unauthorised person to produce unauthorised copies of digital content-carrying recording media. Accordingly, copy-protection techniques have been developed. Most current copy-protection techniques exploit differences in the ways a legitimate player and an unauthorised copying apparatus or “ripper” would access a recording medium in order to defeat copying while preserving good playability.
A legitimate DVD player follows a navigational path on the DVD that is defined by navigational commands and control data carried by the DVD whereas most unauthorised copying apparatus or rippers access the content of a DVD file-by-file or sector-by-sector. One way of frustrating production of good quality or playable copies by a file-by-file or sector-by-sector unauthorised copying apparatus is to include subversive data (that is data that detrimentally affects at least one of the copying process, copy quality and playability of a copy) in a sector that does not form part of the navigational path of the DVD. Such a subversive sector will be ignored by a legitimate player because it does not form part of the navigational path. However, a file-by-file or sector-by-sector unauthorised copying apparatus trying to copy the DVD would encounter the subversive sector and, as a result, would report an error and would be unable to read the DVD.
Current techniques for producing a subversive or unreadable sector include techniques that modify or corrupt the digital sum value (DSV), corrupt inner or outer parity codes or otherwise corrupt the EFM+ (eight to fourteen modulation plus) data stream for that sector in some way. These techniques can, however, cause a physical testing device testing a legitimate copy-protected DVD to report that a copy-protected DVD is unsatisfactory because the physical testing device may detect inner or outer parity errors, jitter, timing errors, radial noise, poor reflectivity or the like when the physical testing device encounters the copy-protected sector(s). Accordingly, physical testing of such copy-protected DVDs requires modification of the software used by the physical testing device to enable such a copy-protected DVD to receive a clean or satisfactory test report. For example “plug-ins” may need to be specially developed to enable the physical testing device to deal with each particular different type of copy-protection.